(a) Field of the Invention
The present invention relates to a variable valve timing device and a control method for the same.
(b) Description of the Related Art
A typical combustion chamber of an automotive engine is provided with an intake valve for supplying the air/fuel mixture and an exhaust valve for expelling the burned gas. The intake and exhaust valves are opened and closed by a valve lift apparatus connected to a crankshaft.
A conventional valve lift apparatus has a fixed valve lift amount due to a fixed cam shape. Therefore, it is impossible to adjust the amount of a gas that is being introduced or exhausted.
If the valve lift apparatus is designed for low driving speeds the valve open time and amount are not sufficient for high speeds. On the other hand, if the valve lift apparatus is designed for high speeds, the opposite is true.
One proposed system to improve efficiency in high and low driving speeds includes cams that have different profiles. At high speeds, a high lift cam that has a high lift profile and increases opening angles of valves is used, and at low speeds, a low lift cam that has low lift profile and decreases opening angles of valves is used.
Another proposed system includes a camshaft timing gear provided with a variable valve timing unit in front of an intake valve camshaft, that changes phase angles of the cams without changing opening/closing valve timing.
However, a variable valve device that uses a variable valve timing unit controls only a phase angle of an intake valve, so efficient control of valve timing is difficult. Alternatively, variable valve timing units must be provided with camshaft phase sensors at both intake and exhaust camshafts. Controlling such a unit is very complicated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.